The presence of near-surface microshrinkage in investment cast metals is an inherent defect in the investment casting process. Investment cast nickel base superalloys are widely used in modern gas turbine engines, but their use in complex-shaped structural applications has been limited by the presence of such near-surface microshrinkage.
Investment casting involves pouring molten metal into a mold produced by surrounding an expendable pattern with a refractory slurry that sets, after which the pattern is removed, usually through the use of heat. Investment casting permits the making of near net shape parts of very precise dimensions and complex angles and shapes as well as parts having intricate internal structure, such as the internal passages of turbine blades or frame struts, with the need for little or no finishing.
Because investment cast nickel base superalloy parts are used for critical structural applications in modern turbine engines, including such applications as engine mounts, turbine frames, turbine blades, frame struts, combustor casings and pressure vessels, they are subjected to repetitive inspection techniques to identify both surface and subsurface flaws. These inspection techniques include radiographic testing, eddy current testing and penetrant testing and their procedures are well known in industry.
These inspection techniques have been successfully employed to detect most surface and subsurface defects in nickel base superalloy parts used in aircraft engines. In certain instances, these techniques alone have been found to be inadequate to locate certain defects, requiring modifications to standard commonly practiced processes or the addition of operations before these processes can be successfully applied. An example of such a modification is described by Fishter et al. in U.S. Pat. No. 4,534,823, in which the patentees describe a method of detecting minute surface cracks on the metal surface of a machined, wrought IN-100 nickel alloy. This method utilizes standard fluorescent penetrant inspection procedures to detect surface cracks after a very small amount of a smeared machined surface layer, about 0.0001 to about 0.0006 inches, has been removed by chemical milling. This method has been useful for increasing the sensitivity of cracks open to the surface of a worked IN-100 surface using the chemicals and reagents described in the '823 patent, by removal of minute amounts of smeared material.
Such processes and standard inspection methods either alone or in combination have been found to be of no value in detecting defects in investment cast parts referred to as near-surface microshrinkage. Near surface microshrinkage is microscopic voiding of material in investment cast parts located close to, but beneath, the part surface which may be microscopically connected to the part surface. Since the investment casting process randomly produces this difficult-to-detect microshrinkage which can reduce the ability of a part to withstand stress levels encountered in service, it is not uncommon for parts made by the investment casting process to be limited to applications in which the operating stress levels are sufficiently low so as not to adversely affect part performance when such defects are present. As a result, parts made by the investment cast process often are not used in applications in which microshrinkage can reduce the ability of a part to withstand the operating stress levels.
Eddy current inspection is a process which has found to be successful for detecting defects open to the surface of a part. In this method, an electric field is generated within the part, and variations in the electric field indicate the presence of a surface defect. In addition to being limited to the inspection for surface defects, eddy current inspection is also slow and time-consuming, particularly for inspection of complex structural castings.
Dye penetrant inspection is another process which can successfully detect indications open to the surface of a part. Dye penetrant inspection procedures are well known in the industry. In dye penetrant inspection, a low viscosity material is applied to the surface of the part under inspection, and through capillary action, is infiltrated into indications open to the surface, such as cracks or porosity. Excess penetrant is then removed from the surface of the part, and a material capable of drawing the penetrant absorbed into the surface openings is applied to the surface of the part. By capillary action, the penetrant material in the openings is drawn back to the surface, thus revealing the location of indications. When using the visible dye process, the high visibility of the dye contrasted against the background of the part reveals the location of the openings. In the fluorescent penetrant process, illumination of the surface with an ultraviolet light causes fluorescence at locations where the penetrant has been drawn back to the part surface. The limitation of this process is that the indications must have a sufficiently large opening to the part surface in order for the penetrant to be drawn into the indications by capillary action.
While near-surface microshrinkage may be microscopically interconnected to the part surface, the surface connections are so small that capillary action cannot draw the penetrant into the microscopic openings, that is, the openings are beyond the capabilities of this inspection method.
Even when the microscopic openings are large enough to allow the penetrant to enter, the resulting penetrant indication is often indistinguishable from other non-relevant indications attributable to surface roughness of the part, thus resulting in an inspection technique that is not reliable in detecting a defect which may degrade part life when used in critical applications.
Ultrasonic inspection is a process which has been successfully used to detect subsurface indications in materials. Ultrasonic inspection utilizes a high frequency sound beam which is transmitted into the part under inspection. In the commonly used pulse echo method, short bursts of ultrasonic energy are introduced into the test piece at regular intervals of time. If the pulses encounter a reflecting surface, such as a flaw in the part, some or all of the energy is reflected. The proportion of energy that is reflected is highly dependent on the size of the reflecting surface in relation to the size of the incident beam. The direction of the reflected beam (echo) depends on the orientation of the reflecting surface with respect to the incident beam.
Reflected energy is monitored with respect to the amount of initial pulse energy as well as the time delay between the transmission of the initial pulse and receipt of the echo pulse. By using the reflection of the sound energy from the back surface of the part as a reference point, both as a time reference and as an intensity-of-reflected sound reference, flaws can be detected and located at depths within the material.
The limitations of this method include an inability to detect material discontinuities that are present in a shallow layer immediately beneath the surface. This region, commonly referred to as the near field region, is generally the location of microshrinkage. Detection of flaws or indications is further complicated by excessive background "noise" resulting from the reflection of the sound waves from coarse grains usually present in cast parts. This background noise masks reflections from actual indications, thus making ultrasonic methods impractical for inspection of castings.
Radiographic inspection is another method used to detect subsurface discontinuities. In this inspection method, the part is exposed to short wave length electromagnetic radiation. Discontinuities may be either areas where there is a lack of material, and hence a variation in thickness, such as porosity or cracks, or where there is a density difference, such as inclusions. Because of differences in the absorption characteristics of the part at the location where these discontinuities are present, different amounts of radiation pass through the part. These different amounts of radiation, when recorded or observed by detectors, such as radiographic film, reflect the presence of the discontinuity. Among the limitations of radiographic inspection are the inability to detect microporosity, microshrinkage and microfissures unless they are sufficiently segregated to yield a differential density sufficient to represent a detectable gross defect. Also, grain refraction of the beam causes false indications on the X-ray film which tends to mask actual defects. Still another limitation includes difficulties with inspecting parts having complex shapes and varying thicknesses. Finally, this technique is relatively expensive.
Because no reliable nondestructive methods exist to detect the near-surface microshrinkage present in investment cast metal parts, it is currently necessary to rely on destructive techniques to produce parts suitable for use in gas turbine applications. By this technique, representative parts are cross-sectioned after hot isostatic pressing and interrogated for the presence of near-surface microshrinkage. Interrogation involves locating defects, including microshrinkage, and evaluating such defects in accordance with known acceptance standards. The presence of near-surface microshrinkage in excess of predetermined amounts in cross-sectioned parts usually indicates a need to modify the investment casting process for making the parts. But typically, less than about five per cent of the volume of any one casting can be interrogated by this technique.
All of the aforementioned inspection techniques, including sectioning, require an understanding of the art of investment casting in order to tentatively identify and isolate the location of microshrinkage in investment cast parts. These suspect locations are then destructively interrogated, by sectioning, or nondestructively interrogated, for the presence of indications. However, only sectioning has been effective in locating the presence of near-surface microshrinkage. But the information obtained by sectioning is limited to the cross-section which is actually interrogated. The randomness of the casting process also results in an unpredictable variation in the location and severity of the near-surface microshrinkage from one casting to the next.
Because nondestructive testing techniques are unable to find near-surface microshrinkage, and because sectioning is unreliable in ascertaining that subsequently produced castings are free of near-surface microshrinkage, no reliable method currently exists to assure that investment cast parts produced for critical turbine engine applications are free from near-surface microshrinkage.